This invention relates to an improved injection molding nozzle and a method of making it in which the components are integrally brazed together with an electrical heating element in a spiral channel around the outer surface and provided with a protective nickel coating.
Casting helical heating elements in an injection molding nozzle is well known in the art. For instance, the applicant's U.S. Pat. No. 4,238,671 entitled "Sprue Bushing with Cast in Heater Element" which issued Dec. 9, 1980 shows a helical heating element cast around a high strength corrosion resistant inner core portion through which the melt passage extends. An improvement in this early method is shown in the applicant's U.S. Pat. Nos. 4,355,460 entitled "Sprue Bushing and Method of Manufacture" which issued Oct. 26, 1982 and 4,386,262 (divisional) entitled "Sprue Bushing with Cast in Electrical Heating Element" which issued May 31, 1983. In these patents, the helical heating element is vacuum case in a copper portion between the inner core portion and an outer sleeve. Further improvements are shown in the applicant's U.S. Pat. Nos. 4,403,405 entitled "Sprue Bushing Connector Assembly Method" which issued Sept. 13, 1983 and 4,446,360 (divisional) entitled "Sprue Bushing Connector Assembly" which issued May 1, 1984 which specifically relate to sealing around the heating element cold terminal using a split washer arrangement. While the nozzles disclosed in all of the above patents ave the advantage that the heating element is cast in copper which rapidly disperses the heat, they have been found to have several disadvantages for certain applications. Firstly, with the increasing demand for high temperature and high pressure applications, it is not possible to provide the required bursting strength without increasing the outside diameter of the nozzles, which is not acceptable for some cavity configurations. Bursting strength minimum requirements of 50,000 psi. are now quite common and this is difficult because the copper portion around the heating element is relatively weak and does not add much strength. Secondly, it is desirable that the coils of the heating element be accurately located in the nozzle. Furthermore, because there is usually more heat loss at the ends of the nozzle than in the middle, it is desirable that the pitch of the heating element vary along the length of the nozzle according to a predetermined pattern, depending upon the application. This has not been found to be possible with these previous nozzles where the central core is inserted into the helical heating element which is then cast in copper.
More recently, in order to overcome some of these and other problems, the applicant has provided a nozzle with a heating element brazed in a spiral channel in the outer surface of the nozzle body. This is shown in the applicant's U.S. Pat. Nos. 4,557,685 entitled "Heated Nozzle for Injection Molding Apparatus" which issued Dec. 10, 1985 and 4,583,284 (divisional) entitled "Method of Manufacture of Injection Molding Heated Nozzle with Brazed in Heating Element" which issued Apr. 22, 1986. However, unfortunately this has been found not be entirely satisfactory because the nickel brazing compound does not flow evenly throughout the spiral channel and thus the contact between the heating element and the body is not uniform throughout its length. Furthermore, this method has the disadvantage that a protective coating is not provided over the entire surface of the nozzle. Also, separate brazing steps are required in the vacuum furnace for embedding the heating element and mounting the insulation bushing and connector sleeve.
Even more recently, the applicant's Canadian patent application Ser. No. 532,677 entitled "Injection Molding Nozzle and Method" which was filed Mar. 20, 1987 discloses an improved method of providing a nozzle which may have a protective outer steel sleeve. However, if minimum outside diameter of the nozzle is important, which is often the case, this steel sleeve is machined off.